There have been a number of published proposals for the use of detectable changes in quantum tunnelling current to measure micro- or nano-order displacements. The displacements may be linked to a physical property to be measured.
The existing proposals may be broadly classified into two groups, a first group in which a measurement tip undergoes lateral movement with respect to another electrode, sometimes referred to as Lateral Tunneling Unit (LTU) techniques, and a second group where a flexible proof mass, for example in the form of a free end of a cantilever may be movable in multi-dimensions with reference to a counter electrode. One significant common feature between the existing proposals is that they all rely on a variation in the distance between an electrode on the measurement tip or proof mass and the counter electrode for the detectable variation in the tunneling current.
Examples of the first group include D Kobayashi et al (‘An integrated Lateral Tunneling Unit”, IEEE 1992, Micro Electro Mechanical Systems 1992, Travemuende (Germany) Feb. 4-7, 1992, p. 214), and H Toshiyoshi et al ('Micromechanical Tunneling Probes & Actuators on a Silicon Chip', IEEE1999, Microprocesses and Nanotechnology Conference 1999, p. 180). An example of the second group is proposed in WO96/21157.
However, since the tunneling current varies exponentially with the distance between the measurement tip or proof mass and the counter electrode, the existing proposals may not be well suited for accurate measurement and monitoring of fine displacements, and in particular of fine incremental displacements.
For example, WO96/21157 discloses a micromechanical accelerometer having a substrate and a proof mass mounted on the substrate. Both are formed of conductive material. The proof mass is a flexible element, having a free end which is a first electrode. It faces a second electrode on the substrate, and circuitry is provided to measure quantum tunneling current between the electrodes when a voltage is applied between them. The gap distance between the electrodes is at an angle to the direction of motion of the free end of the flexure, so that the gap varies as the flexure flexes. In the disclosed accelerometer, the quantum tunneling current thus varies exponentially with the distance between the electrodes.
A need therefore exists to provide a sensor device and method and method of fabricating the same which seek to address the above-mentioned disadvantage of existing proposals.